I think that two or three hands of resin coating, with fairing and a good
cycle of polyurethane paints, will give wood the enaugh protection versus
rot.
Not sure that more resin will give boat more strenght, being the wood
structural. Rigidity is ensured longitudinally from wood strips and
transversally from bulkheads . Anyhow, two external layers of biaxial
reinforcement could be useful for stiffness.
The original projet is for a GRP-sandwich 19' cabin sailboat, displacement
800 kg, net weight about 500 kg, 200kg of ballast.
I am not sure in wood thickness for strip planking: considering
reinforcements, I'm oriented in 1/2'' to save weight but maybe 3/4'' could
be better?
Ciao
Leo
"William R. Watt" wrote:
the weigth of the resin used to encapsulate the wood could make the weight
difference of the wood negligeable. it depends on the resin-to-wood ratio
you are using and that depends on whether the design is using the wood or
the resin for the structural stength.
you have to put a minimum thickness of resin on the wood to effectively
encapsulate it. the required thickness depends to some extent on the
rigidity of the hull. if its a light racing dingy the hull is subject
to large deforming stresses, particularly twist from the mast. if the
resin is too thin it will scratch and crack and let in water and there
goes your wood core.
I don't know of any formulae for resolving the above tradeoffs.
here in Ontario native white cedar (arbor vitae) is used because it lasts
in fresh water. (I don't think it is related to European cedars.) Salt
water is not so much a problem for wood because salt inhibits the
growth of wood rot. look at www.cedarboats.com for varnished cedar-on-oak
work boats used by professional fishing guides.
finally, if the boat is not kept in the water and is protected from rain
then rot is not a concern. However I've seen many retired racing dingys
anchored off summer cottages with the transom flaps left open to keep
them from filling up and sinking, where rot would be a problem for a wood
core boat if it were not well "encapsulated"
Glenn Ashmore ) writes:
Sorry. I was thinking on this side of the pond where solid okoume is
very rare and expensive. In that case okoume could be an option. Just
be sure to encapsulate it extremely well in epoxy because it is rated as
non-durable. Once rot starts it will spread very quickly.
You will not have a problem with strength. Okoume is about equal to
cedar in bending strength and stiffness and slightly better in impact
resistance and crush strength.
Weight will be a consideration with cedar running about 370 Kg/M and
okoume averaging 450 Kg/M. Also okoume is hard on tool sharpness.
Leo wrote:
"Jim Conlin" wrote:
Perhaps I misunderstood the original question. Was it about plywood or
solid okoume?
It is solid okoumè.
I know cedar is better: lighter and more durable .
In Italy (I think so in all the Europe) western red cedar is 1,200 Euro
x
cube meter (sorry, metric, but ratio is the same), much more expensive
than
Okoumè.
Therefore, red cedar I found has high weight, similar to okoumè:
450kg/cu.meter (again, see the ratio).
Cedar should be1/5 less heavy on weight to be good.
For that I'm asking about.
Alternative could be Spruce, light as cedar but I think less durable
than
okoumè.
In any case wood will be 'encapsulated' in coatings, as Ron says, as a
core.
My doubts are on strength of these different woods.
Plywood-lapstrake could be a good solution, but I need a smooth round
bilge
for a modern 19' racer.
What do You think about?
Ciao
Leo
Backyard Renegade wrote:
"Ron Magen" wrote:
However, plywood is the material for 'glued lapstrake' construction.
I did struggle with the post I made because of this and other
possibilities, that I why I said, "for strip building (as we know
it)".
As
Okoume is supposedly prone to rot, this would probably do the job if
it was
'encapsulated'. That is, epoxy, cloth, & finish on the outside with a
few
coats of a quality exterior primer and finish paint on the inside. Or
several coats of a UV Varnish . . . and keep them maintained.
I get lot's of guys who have me deliver S+T boats clear coated cause
the Okoume and Mahogany look so nice, I wonder how long they will stay
clear though;-)
Scotty
"Backyard Renegade" wrote:
Any kind of plywood is not good for strip building (as we know it).
The properties of the plywood make it difficult if not impossible to
bend the strips in such a manner as to follow the lines of a hull.
Scotty from SmallBoats.com
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
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